In the manufacture of integrated circuits, thermal oxidation of silicon is one of the key process steps. Modern processes utilize a small amount of chlorine-bearing vapor (HCl or Cl.sub.2, etc.) along with the oxidant in a carrier gas to enhance the properties of the oxide. The chlorine species getters metallic species (e.g. Na, Fe) in the environment by conversion to a chloride which is carried out as a vapor in the carrier gas exiting the furnace or forms chloride in the oxide thus preventing migration of metallic species. This stabilizes electrical properties of the oxide. Chlorine addition also changes the growth kinetics of the oxide.
One of the more widely used chlorine sources for this process is 1,1,1-trichloroethane (TCA) packaged and sold by the Schumacher unit of Air Products and Chemicals, Inc., Carlsbad, Calif. The TCA chlorine source is flowed into a manufacturing furnace in a carrier gas such as nitrogen together with an oxygen source. The silicon article is heated to a temperature in excess of 800.degree. C. to oxidize the hydrocarbon to form high purity hydrogen chloride according to the equation: EQU C.sub.2 H.sub.3 Cl.sub.3 +XO.sub.2 .fwdarw.3HCl+2CO.sub.2 +(x-2)O.sub.2
An excess amount of oxygen is used to prevent formation of carbon by conversion of the carbon to carbon dioxide. Using an excess of oxygen prevents the starvation of the reaction for oxygen thereby converting all of the TCA to HCl and CO.sub.2. This also prevents chance formation of phosgene, a dangerous gas. Argon may be used in place of nitrogen as the carrier gas.
The TCA can be used in a like manner to clean furnace tubes used in the production of semiconductor devices. Here the quartz tube is held at an elevated temperature with oxygen or steam and a chlorine source flowed through the tube. The chlorine generated reacts with metal contaminates producing volatile metal chlorides and these chlorides are exhausted from the tube with the gas flow. If such metals were allowed to remain in the tube during silicon oxidation, they may create significant defects in the silicon. This result is similar to that in silicon oxidation where such contaminants on or near the surface of the silicon wafers are volatilized or made immobile in the growing oxide. Chlorine sources used in the more critical silicon oxidation are suitable for the less demanding tube cleaning.
TCA has replaced HCl gas since the latter is highly corrosive and has caused extensive damage to facilities and to device yields. TCA produces the same reaction products in the same proportions as HCl gas in a hot oxidizing environment. This is critical to the silicon oxidation process since it allows substitution of TCA for HCl gas without extensive experimentation.
Other chlorine sources have been evaluated. Dichloroethane and, especially, 1,1,2 trichloroethylene have been described in the art as sources of HCl for the chloroxidation of silicon (e.g. R.G. Cosway, S. Wu J. Electronics Soc., 1985, 132, p. 151; C.L. Claeys et al J. Appl. Phys., 1980, 51, p. 6183; and T. Hattori, Solid State Tech, 1982, July p. 83) but, as taught by E.J. Janssens, G.J. Declerck, (J. Electrochem Soc, 1978, 125, 1697), the performance of the precursors is best for compounds where the number of hydrogens equals the number of chlorines. The trichloroethylene, for example, generates an excess of diatomic chlorine when it decomposes, resulting in excessive etching of the silicon substrate during the oxidation. In addition, 1,1,2 trichloroethylene is highly toxic as is carbon tetrachloride a source for Cl.sub.2 described by C.M. Osborn (J. Electrochem, Soc., 1974, 121, p. 809). Janssens et al go on to cite methylene chloride, 1,1,2 trichloroethane and 1,1,1 trichloroethane as possible precursors where the number of hydrogens equals the number of chlorines. However, they conclude on the basis of comparative volatilities, chemical stability, commercial availability, and toxicity that the 1,1,1 trichloroethane is the best candidate. Both the 1,1,2 trichloroethane (tlv 10 ppm) and methylene chloride are regarded as toxic and carcinogenic in animal tests. In addition we have found no significant performance advantage of dichloromethane relative to 1,1,1 trichloroethane in chloroxidation experiments on silicon substrates.
The use of TCA as an HCl source and its application is widely practiced in the semiconductor industry. The typical process is similar to that described in S. Wolf, R.N. Tauber, Silicon Processing for the VLSI Era, Volume I, Lattice Press 1986, p 215-216, where the temperature would be greater than or equal to 850.degree. C. The thermodynamic studies of Janssens et al have demonstrated that oxidation of TCA should be complete even at 700.degree. C., however, it has been reported that the gas phase composition of TCA at 800.degree. C. is not at the equilibrium composition under conditions similar to those practiced in the chloroxidation process (J.R. Flemish, R.E. Tressler, J.R. Monkowski J. Electrochem Soc., 1988, 135, 1192).
Current oxidations are carried out at temperatures greater than 800.degree. C. The need for more demanding control over the thickness and quality of thermally grown oxides requires a reduction in the temperature under which the reaction is carried out. The high stability of TCA becomes a disadvantage as the temperature declines, because of the risk of incomplete combustion resulting in the incorporation of carbon in the growing oxide films.
Although TCA has been a useful additive for both chloroxidations and tube cleaning, new trends in processing conditions towards lower temperatures as well as environmental concerns have turned the high chemical stability of TCA into a disadvantage. The high stability of the compound upon environmental exposure is advantageous in the handling of the material, but ensures that the compound is able to reach the upper atmosphere before it decomposes. Breakdown results in the generation of free chlorine radicals which can destroy the earth's protective ozone layer. TCA is currently being phased out for all nonfeedstock commercial applications in the United States by the Environmental Protection Agency.